The purpose of an analog switch is to alternately pass or block a voltage signal which is allowed to vary with time, but for proper operation of the switch, the analog voltage must not range beyond a negative limit which we call V.sub.min and a positive limit which we call V.sub.max. Examples of such allowable analog ranges might be (1) V.sub.min =-15 V, V.sub.max =+15 V; (2) V.sub.min =0 V, V.sub.max =+200 V; or (3) V.sub.min =-100 V, V.sub.max =-10 V. In the case of the switched signal lying outside of the range for which the switch was designed the expectation is that the switch might malfunction or even be damaged.
A typical analog switch based on field effect technology is shown in FIG. 1. A complementary CMOS pair 14 consisting of transistors 15 and 16 is capable of switching signals having a range of -15 volts to +15 volts which are received on line 11. The switched signal is transmitted out on line 12. When a high actuating signal is applied to input line 13, inverter 10 transmits a signal to the gate of the p-channel enhancement transmitter 15 to render it conductive. Simultaneously, the switching signal is communicated to the gate of n-channel enhancement transistor 16 to render it conductive. Any analog signal impressed on line 11 is then passed through the complementary CMOS pair 14. When a low signal is introduced on line 13, the gate of n-channel transistor 16 is kept low so that no signal is transmitted. Similarly, the inverter 10 delivers a high signal to the gate of p-channel transistor 15, thereby rendering it nonconductive. This standard configuration is available as Intersil Part No. IH 5040 or as Analog Devices Part No. AD 7501. It is able to switch analog signals up to the limits of the rails of the power supplies, e.g., -15 V to +15 V, but cannot be used to control signals beyond the rails. In line with the limitations of analog gates based on field effect transistors, as described above, it is difficult to improve this standard analog switch for voltages much higher than several tens of volts because:
(1) the gate oxide is exposed to the full power supply voltage so that if the power supply range is increased to accommodate higher analog signal ranges, then the gate oxide must be grown inordinately thick;
(2) it is difficult to make the n-channel transistors with sufficiently high breakdown since they are fabricated in a p-well; and
(3) the driving inverter 10 must be able to operate through the full analog range, and because of the inability to fabricate the n-channel transistors as described in item (2), this is difficult to achieve in a monolithic format.
High voltage analog switching has been previously accomplished by devices such as mechanical reed relays and optically coupled high voltage semiconductor circuits. Because of the problems described in the previous paragraph, high voltage analog switches have not generally been available as semiconductor components. However, there exists an expanding requirement for high voltage analog switches in modern electronic equipment. In particular, it is desirable to extend the replacement of mechanical and electromechanical components by semiconductors to the high voltage domain. This would allow size, weight and power requirements to be further reduced. Applications for semiconductor-based high voltage analog switches include telecommunications equipment, test equipment and high voltage displays such as electroluminescent, vacuum fluorescent and plasma displays. For such applications, the switches would be required to withstand voltage differentials of up to 400 volts or more.
As described previously, the prior art circuits for high voltage switching have required, in general, the use of two power supplies to define the limits of the switch; these limits are typically called the "rails". Thus, to switch an analog signal which could vary in the range between K.sub.1 volts and K.sub.2 volts, where K.sub.2 is more negative than K.sub.1, e.g., where K.sub.1 =V.sub.max and K.sub.2 =V.sub.min, one would need two power supplies. The first would generate K.sub.1 volts and the second would produce K.sub.2 volts. Thus for high voltage bi-directional switches, one would need in general two power supplies of high voltage rating. In the special case where either K.sub.1 or K.sub.2 is close to ground potential, then only one power supply of high voltage would be required. For example, if the voltage range were -10 volts to +400 volts, then one high voltage supply would be necessary to generate the voltage level of 400 volts. This requirement for power supplies of high rating adds to the cost and complexity of systems employing the conventional circuit configurations.
It is therefore an object of the present invention to provide a bi-directional high voltage switch which can be fabricated with semiconductor components.
It is another object of the present invention to provide a monolithic solid state bi-directional high voltage switch which can be made using conventional semiconductor technology.
It is an additional object of the present invention to provide a high voltage analog gate which requires its rail voltages to be set with respect to only the more negative limit of the analog voltage range as opposed to setting one rail with respect to the negative limit and the other rail with respect to the positive limit.
It is an additional object of the present invention to provide a high voltage switch which in the special case of the analog voltage limits being zero volts and a high positive voltage, neither of the rails need be of high voltage. In this case, one rail is a simple ground and the other rail requires a low voltage negative power supply.
It is a further object of the present invention to provide a high voltage analog switch which is implemented by using the inherent electrical properties of both enhancement and depletion transistors.
It is a further object of the present invention to provide a high voltage analog switch which is implemented using two types of high voltage transistors in addition to conventional CMOS. The first type of transistor is the vertical DMOS commonly known by a tradename "HEXFET". The other type of transistor is lateral conduction offset gate transistor. The above components are integrated into a monolithic circuit using the dielectric isolation technique.